This invention relates to screen stencil exposure apparatus.
Making stencils for stencil screen printing typically involves the steps of coating a screen with a photo responsive emulsion, drying the coating, overlaying the coated fabric on a stencil pattern positive, exposing pattern areas of the emulsion coating to light through the stencil pattern positive, and subsequently developing the exposed emulsion to cause openings in the screen in the pattern desired. Using this prepared screen, stencilling fluid can be forced through the openings as with a squeegee in a conventional manner to selectively coat the underlying stock.
Apparatus for making these stencils normally includes a translucent, and preferably transparent, surface as of glass, on which the positive is mounted and against which a chase mounting the emulsion coated screen is placed. The periphery of the area to be exposed is sealed, and suction is used to draw the positive and the screen together for accurate exposure.
To form this peripheral seal, there is one technique wherein a rubber tube is recessed into a peripheral groove specially routed into the support platform around the translucent surface. Unfortunately, this limits the stencil area to basically one size. Thus, each different size stencil involves a different exposure apparatus.
Another technique involves manually placing a peripheral rubber tube around the stencil area and then squeezing the stencil screen frame to the transparent surface. This has obvious limitations including the necessity of carefully positioning the tube each time, the lack of positive dimensional control of the seal, and the necessity of using a generally horizontal work surface so the tube will stay in place.
The technique employed on vertical exposure units is usually that of placing a rubber blanket over the apparatus and drawing a vacuum. This is cumbersome and not very versatile.